Java HashMap Class

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The Java Collections Framework includes HashMap which belongs to the java.util package. HashMap stores data in key-value pairs where keys are required to be unique while values are permitted to have duplicates. A hash table provides the underlying structure for this collection and enables quick operations for inserting, deleting, and finding elements.

Key Features

  • Stores key-value pairs
  • Keys must be unique
  • The data structure permits a single null key while supporting multiple null values.
  • Unordered – does not maintain insertion order
  • Non-synchronized (not thread-safe)

Syntax

HashMap<KeyType, ValueType> map = new HashMap<>();

Example

HashMap<Integer, String> students = new HashMap<>();

Example

 import java.util.HashMap;
import java.util.Map;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> employeeMap = new HashMap<>();

        // Adding key-value pairs
        employeeMap.put(1, "Alice");
        employeeMap.put(2, "Bob");
        employeeMap.put(3, "Charlie");

        // Access and modify
        System.out.println("Employee 2: " + employeeMap.get(2));
        employeeMap.put(2, "Bobby");

        // Iterating using entrySet
        for (Map.Entry<Integer, String> entry : employeeMap.entrySet()) {
            System.out.println(entry.getKey() + " -> " + entry.getValue());
        }

        // Removing an entry
        employeeMap.remove(3);

        // Check contents
        System.out.println("Has key 1? " + employeeMap.containsKey(1));
        System.out.println("Has value 'Charlie'? " + employeeMap.containsValue("Charlie"));

        // Clear the map
        employeeMap.clear();
        System.out.println("Is map empty? " + employeeMap.isEmpty());
    }
}
Run ›

Note

  • The hashCode() and equals() methods maintain key uniqueness within this data structure.
  • Custom objects that serve as map keys require proper implementation of both hashCode() and equals() methods.
  • Use LinkedHashMap for insertion-order retention.
  • Use TreeMap for sorted key order.
  • Use Collections.synchronizedMap() or ConcurrentHashMap for thread-safe versions.

HashMap vs TreeMap vs LinkedHashMap

Feature HashMap TreeMap LinkedHashMap
Order No order Sorted keys Insertion order
Null Key Allowed (1) Not allowed Allowed (1)
Performance Fastest Slower Medium
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